This invention relates to optical devices and, more particularly, to the fabrication of an optical device such as a distributed-feedback (DFB) laser that includes a phase-shifted grating.
Gratings are included in a variety of discrete and integrated optical devices of practical importance. For example, semiconductor lasers incorporating a grating to provide distributed feedback have emerged as the most promising candidate for achieving single-longitudinal-mode operation. In a DFB laser, the period of a first-order grating for a 1.55-micrometer (.mu.m) wavelength laser is only about a quarter of a micrometer. Conventional contact or projection photolithography is not feasible for realizing such fine grating structures. But fine-pitched simple gratings, without a phase shift, have been made by standard holographic techniques. In accordance with such techniques, the interference of a laser beam with itself results in exposing a layer of photoresist with a light intensity characterized by a sinusoidal distribution.
A DFB laser with a simple grating exhibits a two-fold degeneracy. As a result, stable single-mode operation is not always achieved. But this degeneracy can be eliminated by introducing a quarter-wave phase shift in the grating (which corresponds to a 180-degree phase shift in a first-order grating), as described by H. A. Haus et al, "Antisymmetric Taper of Distributed Feedback Lasers," IEEE J. Quantum Electron, QE-12, 532(1976) and K. Utaka et al, "Analysis of Quarter-Wave-Shifted DFB Lasers," Electronic Letters, 20, 326 (1984).
Various techniques are known for making phase-shifted gratings suitable, for example, for use in DFB lasers. One technique utilizes direct-writing electron-beam lithography [see K. Sekartedjo et al, "1.5 .mu.m Phase-Shifted DFB Lasers for Single-Mode Operation", Electronic Letters, 20, 80 (1984)]. But this technique suffers from the disadvantages of long writing times, grating period nonuniformity, high equipment cost, and possible stitching and address digitizing errors.
Other proposed techniques for making phase-shifted gratings are based on a holographic grating with either the use of an appropriately patterned phase retardation plate [F. Koyama et al, "1.5 .mu.m Phase Adjusted Active Distributed Reflector Laser for Complete Dynamic Single-Mode Operation," Electronic Letters, 20, 391 (1984)]or the simultaneous use of positive and negative photoresists, with the reversal between the positive and negative images producing the desired 180-degree phase shift [K. Utaka et al, ".lambda./4-Shifted InGaAsP/InP DFB Lasers by Simultaneous Holographic Exposure of Positive and Negative Photoresists," Electronic Letters, 20, 1008 (1984)]. But each of these known techniques also suffers from one or more disadvantages such as: requiring complex processing, producing an uneven grating structure, exhibiting a relatively wide phase-shift transition region or producing a phase shift that is dependent on the grating period.
Accordingly, continuing efforts have been directed by workers skilled in the art aimed at trying to devise other ways of making a phase-shifted grating. It was recognized that these efforts, if successful, had the potential for providing a basis for making optical devices such as DFB lasers characterized by lower cost and improved performance.